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Help needed with Richie Burnett's SSTC

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LAN_403

Doubl3 Helix

Mon Feb 09 2009, 02:58AM

Registered Member #1548 Joined: Mon Jun 16 2008, 09:22PM
Location: Ohio
Posts: 61

Hello all,

I have decided to build Mr. Burnett's SSTC as my first SSTC. I have most of it done. I'm using a 4 inch diameter piece of PVC pipe as my secondary former. I have about 27 inches of 24 AWG winding. I'm making the primary in the immediate future. I have the control stage done, and most of the full bridge. Where i'm having issues is with the gate drive transformers. They are supposed to be 1:1:1 16 turn GDTs on a toroidal core. He says on his page:

This isolation transformer has two secondary windings wound in opposite directions to drive the gates of each MOSFET.

For the cores I used these cores from the Electronic Goldmine. I wound the primary and one secondary as a twisted pair from a Cat5 cable. The other secondary I wound with a single strand from the Cat5 cable. I'm fairly certain that I wound the latter in opposition to the primary, but on my scope the outputs of the secondaries are the same. Shouldn't the square waves of the secondary coils be inverted from one another?

I can probably get pics of each output of the GDTs tomorrow if that would help in diagnosing this potential problem.

For reference, here is the schematics that are provided:

Thank you for your time, any help will be appreciated.

Frosty90

Mon Feb 09 2009, 07:42AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Just reverse the connections on one of the secondaries. Every time i've wound a gdt, i am certain i connect them/wind them crrectly the first time, but i havnt managed to get it right first go yet!

GeordieBoy

Mon Feb 09 2009, 11:35AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Just twist three wires from the CAT5 cable together and wind them through the core you have (which is a suitable ferrite material.)

Then mark the start of the primary and one of the secondaries with a dab of paint (permanent marker) Mark the start of the other secondary at the opposite end of the bunch of wires. That way both of the secondaries start at opposite ends of the bunch. The two secondaries will then produce complementary voltage waveforms.

When you view the two secondary voltages on a scope you need to make sure that you do the test correctly. You need to use a dual channel oscilloscope with the trigger set to channel 1 only, and use the "chop" mode to display both channels simultaneously. Only then will the two waveforms be displayed correctly, and truely look complementary.

If you have the scope set up to trigger wrongly or you have selected "alt" instead of chop it can force both waveforms to look like they are in phase by triggering on the rising edge of each regardless of whether they are in phase or not. If swapping the polarity of the connections to one of the secondaries doesn't invert one of the waveforms, then you don't have the scope set up correctly to perform the measurement.

Also you must do the test on the bare GDT, not with it installed in a SSTC with power applied. At best this will confuse the results, at worst it will damage the scope.

-Richie,

Doubl3 Helix

Mon Feb 09 2009, 10:57PM

Registered Member #1548 Joined: Mon Jun 16 2008, 09:22PM
Location: Ohio
Posts: 61

Thank you very much for your replies! They are very much appreciated. I will try out the suggestions either today or tomorrow, and post how everything goes. Thanks again for the help!

Edit: It worked like a charm, turns out I need to take a crash course on o-scope usage!

Well anyway, I just hooked up both outputs to my scope, and turned on chop, and what do you know! Exactly what I wanted to see! Well at least I don't need to rewind them...

Here's a pic of the waveform...

Gdtwaveform

Is this decrease in voltage normal, or not? If abnormal, should it matter much? Will it still be sufficient to drive the gates of my FDPF20N50 MOSFETs? (Datasheet)

Rise time is approx. 110ns
Fall time is approx. 60ns

As for ringing, there is about 150ns of ringing after the rise, and about 80ns of ringing after the fall...is this acceptable?

Here are my actual GDTs...

Gdts

~Doubl3 Helix

Doubl3 Helix

Sun Feb 15 2009, 06:28PM

Registered Member #1548 Joined: Mon Jun 16 2008, 09:22PM
Location: Ohio
Posts: 61

Hey guys im back again...so, first things first. I got the coil working! Only one problem, I can't seem to find resonance! I think I have hit a harmonic at 867kHz or so, im only getting like 2 inch streamers there. Me secondary is 27 inches of winding on a 4.25 inch OD PVC pipe, wound with 24 AWG wire. There's no way that such high of frequency is my fundamental...right? Also I have been down sweeping between 112kHz to 220kHz to find it, but that entire range is giving about 1 cm corona. Could interference from the coil be causing my driver to not drive properly, because I noticed that if i moved my hand over the driver, I get more discharge. Could this also be an artifact of the feedback of the mains supply?

P.S. I am also using full wave rectified mains, not half wave like in the schematic.

GeordieBoy

Thu Feb 19 2009, 10:40AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> Is this decrease in voltage normal, or not? If abnormal, should it matter much?

By decrease in voltage, do you mean the little step that is half way along the tops and bottoms of the drive waveform? The steps you are seeing half way through each pulse are due to the magnetising inductance of the GDT. The triangular magnetising current switches between the gate-drive circuit and the diodes protecting the gate-drive circuit as it ramps up and down. It is this behaviour that results in steps of about 0.7v during the pulses. This behaviour is perfectly normal when the GDT isn't actually driving anything. It will likely dissappear when the GDT is actually driving MOSFET gates.

The overall amplitude of 19 v pk-pk is okay provided it doesn't drop when under load. Something like 30v pk-pk would be better to ensure the FETs are turned on fully.

> Will it still be sufficient to drive the gates of my FDPF20N50 MOSFETs? (Datasheet)

You'll have to check the details, but like;ly yes.

> As for ringing, there is about 150ns of ringing after the rise, and about 80ns of ringing after the fall...is this acceptable?

As long as it doesn't cross the turn-on threshold of the MOSFET a _LITTLE BIT_ of ringing right after the edges isn't a problem. Excessive ringing will likely turn into full-blown HF oscillation under power. You need to measure gate-source voltage waveforms when actually driving the MOSFETs though, not just the output of the GDT when connected to nothing.

-Richie,

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